Fluorine-containing ion exchange membranes have excellent heat resistance, chemical resistance, and the like, and are used in various applications as electrolytic diaphragms for alkali chloride electrolysis, ozone producing electrolysis, fuel cells, water electrolysis, hydrochloric acid electrolysis, and the like.
Among these, in alkali chloride electrolysis where chlorine and alkali hydroxide are produced in particular, the ion exchange membrane process is primarily used in recent years. The ion exchange membrane used in the electrolysis of alkali chloride is required to have various characteristics. For example, required are characteristics such as electrolytic performance that electrolysis can be performed at a high current efficiency and a low electrolytic voltage, and the concentration of impurities (such as alkali chloride in particular) contained in the produced alkali hydroxide is low, as well as membrane strength and like characteristics that the membrane strength is so high that no damage is incurred during membrane handling and electrolysis. In addition, while the electrolytic performance and the membrane strength of an ion exchange membrane are in a trade-off relationship, there are demands for the development of an ion exchange membrane having both characteristics at high levels.
Patent Literature 1 discloses an ion exchange membrane consisting of at least two layers, i.e., a fluorine-containing polymer layer having a sulfonic acid group and a fluorine-containing polymer layer having a carboxylic acid group.